Abstract: Tests were carried out to evaluate the toxicity of Chenopodium ambrosioides (Chenopodiacea) leaf against the 0-2 d old adult, 1st, 2nd, 3rd and 4th instar larvae of Anopheles gambiae (Diptera: Culicidae) as petroleum ether extract and essential oil respectively at ambient laboratory temperature. Thirty larvae at each instar stage were exposed to 50, 100, 250, 500, 750 and 1 000 mg/L of extract and essential oil in water, and the number of dead larvae recorded at 24 and 48 h post-exposure. Thirty adults were exposed to C. ambrosioides oil vapour in air tight rectangular glass cages at 0.8, 1.6 and 2.4 μL/L and mortality recorded 24 h after exposure. The results showed that the test extract and essential oil were toxic to all  larval stages and adults of An. gambiae. The median lethal concentration after 48 h exposure (48 h-LC₅₀) values show that the extract was most toxic to the 1st instar larvae (14.89 mg/L) followed by the 4th instar (18.90 mg/L) and least toxic to the 3rd instar (183.77 mg/L). The 48 h LC₅₀ values showed that the essential oil was most toxic to the 4th (36.62 mg/L) followed by the 1st instar (90.75 mg/L) larvae. The computed LC₅₀ value of C. ambrosioides oil against adult An. gambiae was 1.01 μL/L. The study highlights the potential of C. ambrosioides for the control An. gambiae.

Key words: Malaria, Anopheles gambiae, Chenopodium ambrosioides, insecticidal activity, bioassay, median lethal concentrations, essential oil, petroleum ether extract